1. FIELD OF THE INVENTION
The present invention relates to methods and apparatus for testing ceramic members, and particularly to the proof-testing of ceramic members, for example ceramic valve members for engines, for the purposes of quality assurance. Although the invention is particularly described herein in relation to testing of ceramic engine valve members, it is not limited to this application and is generally applicable to ceramic parts and components.
2. DESCRIPTION OF THE PRIOR ART
The use of ceramic components to replace metal parts, for example, in internal combustion engines, is becoming increasingly common. Ceramics provide good wear resistance, strength at high temperatures, and lower mass than metal parts, and their use can reduce friction and fuel consumption. One example is the use of ceramics for engine valve members, which are subjected to severe stresses during operation of the engine when the valve, which is exposed to gases at high temperatures, opens and closes for example 6,000 times per minute. A ceramic material such as silicon nitride can withstand these stresses, but defects may be introduced during the production of the valve member which could lead to failure of the valve in use if they are not detected.
It appears that attention has not yet been paid to the problem of quality assurance of ceramic valve members, or other ceramic parts, which are made by mass production methods. Rigorous quality assurance is required, in order that ceramic components shall reach the same standard of reliability as has become commonplace for components made of metal. Although understanding of the behaviour of ceramic materials is increasing rapidly, it has not reached the same level as that of metal components. Inspection techniques for ceramic materials and components which have been described in the prior art are generally not adapted to quality assurance in mass production. In mass production, the inspection time available may be only a few seconds for each component tested, in order not to slow down the production process. Furthermore, the test should be capable of providing an assurance of quality for the designed working life of the component, which for example is the same as the intended life of an automobile engine in which the component is fitted.
Mechanical testing methods of ceramic materials and components have of course been described, such as bending tests. JP-A-3-13842 describes a rotary bending test and a tensile test for a ceramic valve member. Also described in the prior art are thermal resistance tests, such as a thermal shock test, which involves rapid cooling from high temperature. Spot-heating by lasers or other heat sources has been used to cause crack propagation in test disks. It is also well known to test a ceramic material by subjecting it to mechanical stress while it is maintained at a predetermined temperature.
An article "Silicon nitride turbo-charger rotor for high performance automotive engines" by T. Shimizu et. al., SAE Technical Paper No. 900656, also published in SAE Proceedings No. SP-823, pages 163-175, describes the development and testing of a ceramic turbocharger rotor, including durability testing under conditions more severe and longer than those in actual engines. Proof testing of mass-produced rotors is not discussed.
Methods have also been developed for nondestructive testing of ceramic materials and components, in order to detect small defects. Such techniques are x-ray imaging, ultrasonic scanning and fluorescence dye penetration, but in the testing of mass produced articles, these techniques alone are not practical or are not capable of detecting very small defects, e.g. less than 100 .mu.m in size.
To summarise therefore, conventional testing methods for ceramic materials and components have generally been for the purposes of research, and the requirements of testing for quality assurance in mass production have not been addressed.